How to Calculate Breaker Size for A Motor

Properly sizing a circuit breaker for a motor is crucial for safety and efficiency. This guide explains the calculation process, provides a free online calculator, and offers practical advice for electrical installations.

Why Proper Breaker Sizing Matters

Selecting the correct breaker size for a motor is essential for several reasons:

Safety: Oversized breakers can cause electrical fires, while undersized ones may trip frequently.
Motor Performance: Proper sizing ensures the motor operates efficiently and lasts longer.
Code Compliance: Electrical codes require proper breaker sizing to prevent hazards.

Important Note

Always consult a licensed electrician for critical installations. This guide provides educational information but should not replace professional advice.

Calculation Method

The standard method for calculating breaker size for a motor involves determining the motor's full-load current and applying the appropriate multiplier based on the motor's service factor.

Formula

Breaker Size (Amps) = (Motor Full-Load Current × Service Factor) + 25%

Where:

Motor Full-Load Current: The current drawn by the motor at full load (typically found in the motor nameplate)
Service Factor: A multiplier based on the motor's service factor (typically 1.15 for continuous duty motors)

The 25% addition accounts for inrush current, which is higher than the motor's continuous operating current.

Step-by-Step Calculation

Locate the motor nameplate to find the full-load current rating.
Determine the motor's service factor (typically 1.15 for continuous duty motors).
Multiply the full-load current by the service factor.
Add 25% to the result to account for inrush current.
Round up to the nearest standard breaker size (e.g., 15, 20, 30, 40, 50 amps).

Example Calculation
Step	Value	Notes
Motor Full-Load Current	10 amps	From motor nameplate
Service Factor	1.15	For continuous duty motor
Step 3 Calculation	10 × 1.15 = 11.5 amps	Multiply current by service factor
Step 4 Calculation	11.5 × 1.25 = 14.375 amps	Add 25% for inrush
Final Breaker Size	20 amps	Rounded up from 14.375

Common Mistakes to Avoid

Using the motor's nameplate current without applying the service factor
Ignoring the 25% inrush current addition
Selecting a breaker size that's too small for the motor's starting current
Not considering the motor's voltage rating when selecting the breaker

Warning

Never use a breaker that's smaller than the calculated size. This can cause overheating and potential fire hazards.

Practical Tips

Always use a breaker that matches the motor's voltage rating
Consider the motor's starting current if it's a large motor
For motors with variable loads, use the maximum expected load for calculations
Document all calculations and breaker selections for future reference

Frequently Asked Questions

What happens if I use a breaker that's too small for the motor?: Using a breaker that's too small can cause overheating, frequent tripping, and potential fire hazards. It can also damage the motor and other electrical components.
Can I use the same breaker size for all motors?: No, each motor has specific requirements based on its full-load current, service factor, and voltage rating. Always calculate the proper breaker size for each motor.
What is the service factor for a motor?: The service factor is a multiplier that accounts for the motor's operating conditions. Common values are 1.15 for continuous duty motors and 1.75 for intermittent duty motors.
How do I find the full-load current for a motor?: The full-load current is typically found on the motor's nameplate, which is usually located on the motor housing or frame.
What if my calculated breaker size isn't a standard size?: Always round up to the nearest standard breaker size. For example, if your calculation results in 14.375 amps, you would select a 20-amp breaker.